Using a potentiometer for reading the analog value shows that the values are not linear compared to the position of the potentiometer.
This was reported on discord

In the actual code a pin for reading analog values is initialized with pullups enabled. When connecting a potentiometer to the pin this internal resistor is always parallel to the "upper" part of the potentiometer. The internal pullup resistor is ~35kOhm.
The user who reported this issue used a 500kOhm potentiometer. If the pot is on the "upper" or "lower" end, the correct value is measured. An example for the mid position would be:
"upper" and "lower" part of the potentiometer is 250kOhm, parallel to the "upper" part is 35kOhm, the resulting value is 31kOhm which far away from the 250kOhm for mid position and a high non linearity.
For a 10kOhm Potentiometer it's quite better, the deviation in the mid position is "only" 3%. But it could be still visible espacially for closed loop controls of motorizes axis.